It is desirable for voltage regulators used with portable battery-powered devices to have low output noise and a high Power Supply Rejection Ratio (PSRR). To meet these requirements in the circuit design of a regulator, it is common to use a high value of RC filter to get low noise performance for the reference voltage and thus low output noise for the regulator. High value of RC filters can be implemented via an on-chip resistor and an external bypass capacitor. The bypass capacitor can also improve PSRR at low frequency (below 10 KHz), but not high-frequency PSRR. By supplying the reference and part of the error amplifier with its own output voltage of a regulator (with Vout>1.8V), the PSRR can also be improved. This method, however, may not be used for low output voltage options (with Vout<1.8V). Also, for high frequency (over 10 KHz), the improvement in PSRR may still not be enough.
Currently, high PSRR LDO regulators, such as MIC5305 (with 150 mA maximum load from Micrel Semiconductor and claimed as the industry's highest PSRR LDO regulator) and TPS793xx (with 200 mA maximum load from TI), may offer over 70 dB PSRR with frequencies below 10 KHz. By using a 100 nF external bypass capacitor and a 1 μF output capacitor, the PSRR of MIC5305 may be 85 dB at 1 KHz, 65 dB at 10 KHz, and 48 dB at 100 KHz. If a 10 nF external bypass capacitor and a 10 μF output capacitor is used, TPS793xx may have a PSRR value of nearly 70 dB from 100 Hz to 10 KHz and 43 dB at 100 KHz.